In my effort to purge myself from all things Microsoft, I'm looking for recommendations for a free, non-Microsoft, email client. I'd like something secure that doesn't run out to "momma" and report anything back to the NSA or anyone else. I'd be coming from Microsoft Mail on Windows 7 (I already have my email client for Linux), and would like to be hopefully be able to download the software and have it recognize what I already have set up and it would automatically configure itself and import all my mail, but this isn't required (but email importation is a MUST). I'll mainly just be doing POP3, but IMAP would be nice.

I know Mozilla Thunderbird is popular, and in lieu of any really awesome alternatives that I haven't heard of (and honestly, I haven't looked lately), I may just go with that. I've read some mixed reviews about DreamMail, but would rather have some OCN input on that.

I use and recommend Mozilla Thunderbird. It is easy to set up and use.

I will add that RichiRichX is correct about your privacy, though; all e-mails are run through multiple scanners (mostly spam detectors) while in transit between the sender and the recipient; so the level of privacy your e-mail client itself provides is quite irrelevant.